230 J.J. H. TEALL——PETROLOGICAL NOTES ON 
Site) in. oe: eee rotate Eee Hikos 
Alumina) 4.0 Se eee 17°61 
Herrous;oxdene ret eee 12°04 
LAM # accake Eee pon ee 9-65 
Macnesian eon) hanna ee tl: 5°68 
Potash ree guitar ny cas 1:40 
Sod esa) eter cs amkin, os anaicac 0-56 
Carbonic aciduy asec oe Wess 
99°82 
About 2 miles to the north of the Hett dyke is another dyke 
of almost identical composition (see Mr. Bell’s paper, p. 546); 
and between the two there occurs, at a depth of about 60 fathoms 
‘beneath the surface, a horizontal sheet of similar material, which 
was proved, by the mining operations of Messrs. Bell Brothers, 
to cover an area of at least 15 acres, and to be in one place 19°75 
feet in thickness. Mr. Ernest Bell forwarded a piece of this rock to 
Dr. Trechmann; and to the latter gentleman I am indebted for the 
specimen on which the following statements are based. The rock is 
identical in appearance with the more coarsely crystalline varieties 
of the Hett dyke. Microscopically, also, there is the closest resem- 
blance, the principal difference being the almost entire absence, in 
the case of the intrusive sheet, of any interstitial matter. The spe- 
cific gravity of my specimen is 2°96, and the chemical composition 
of the sheet, as given in Mr. Bell’s paper, is:— 
SLC RR erty eh pee 51:90 
AN UTTAR Sea eae 15°46 
Herroustoxaden ela ante 12°87 
WTI Ss, Mea hc td oy ES 13°80 
Maemesia | Serius eee 4°02 
Potten eo ae aes eee 1-2 
SOG tL Soh Oa es I 0-48 
Carhonicvacidi sae ae 1-02 
100-76 
Of what age are these dykes? To this question no definite answer 
can be given. There is no evidence that either of them penetrates 
the Magnesian Limestone, although they are known to occur in the 
Coal-measures beneath. Prof. Sedgwick remarks that the Hett 
dyke can be traced to Quarrington Hill, but that it does not pass 
up into the Permian ; and from this fact he is inclined to regard it 
as of Paleozoic age. It is worthy of note that the rock of which 
these dykes and the accompanying “sill” are composed differs in a 
marked manner from that of the Cleveland dyke; it is more basic 
in composition, of higher specific gravity, and much more largely 
composed of crystalline constituents. In all these points it ap- 
proaches in character the rock of the Great Whinsill, with which I 
am strongly inclined to regard it as contemporaneous. 
